Packet access method in a cellular digital radio-communications system

ABSTRACT

A method of performing cellular digital radiocommunications in a TDMA system implemented within a network of geographical cells through which mobile stations (SM1 to SM22) travel, a base station being associated with each of the cells, a mobile station communicating via the base station that is associated with the cell in which the mobile station is located. Each each of the frames in the up transmission direction, from the mobile stations to the base station, contain at least one shared time slot (IT6) which is allocated to at least two distinct mobile stations (SM7 to SM22). For each shared time slot (IT6), the system has a list (31) of the active mobile stations, an active mobile station being a mobile station to which the shared time slot is allocated, and which satisfies at least one predetermined selection criterion. For, each frame in the up transmission direction, the system associates each shared time slot with a mobile station chosen from the list of the active mobile stations associated with the shared time slot, the choice being made by using a predetermined strategy.

The invention relates to data radiocommunications. The invention isparticularly applicable to cellular digital radiocommunications systemsfor mobile stations, such as systems designed in compliance with the GSM(Global System for Mobile communications) public radiotelecommunicationsstandard.

In general, a cellular digital radiocommunications system is implementedwithin a network of geographical cells through which mobile stationstravel. A base station is associated with each cell, and a mobilestation communicates via the base station that is associated with thecell in which the mobile station is located.

In this type of system, the calls in stable mode are conveyed overtraffic channels (TCHs) in both transmission directions, namely in thedirection from the mobile station to the base station (up direction),and in the direction from the base station to the mobile station (downdirection).

In addition, this type of system implements time-division multiplexingusing a technique referred to as "TDMA" (Time Division Multiple Access).

In known manner, the TDMA technique consists in dividing up time intoframes of fixed and predetermined duration, which frames are themselvesdivided up into time slots. Thus, the signals conveyed by the system areorganized in frames, and each time slot of a frame corresponds to atraffic channel on which a call in stable mode can be conveyed in onedirection.

On transmission, in each frame in the up direction, a mobile stationtransmits data during the time slot which is allocated to it only. Ineach frame in the down direction, the base station transmits data ineach of the time slots so as to communicate simultaneously with aplurality of mobile stations, each of which is associated with arespective one of the time slots.

On reception, each mobile station knows how to extract the time slotwhich is addressed to it from each frame in the down direction.Similarly, the base station knows which mobile station is associatedwith each of the time slots in the frames that it receives.

In conventional systems, of the type described above, a time slot isallocated to a mobile station for the entire duration of a call. Thismay be referred to as "circuit mode" allocation.

Such circuit mode allocation suffers from a major drawback, namely thatthe transmission resources are under-used. During a call between amobile station and a base station, these two entities do not transmitdata continuously. In other words, there are time slots during which nodata is transmitted.

A known solution to remedy that drawback consists in using at least oneof the time slots in each frame in packet mode. In packet mode, a timeslot (i.e. a traffic channel) in the up direction is shared between aplurality of mobile stations. Thus, a mobile station which wishes toshare a traffic channel with other mobile stations sends an allocationrequest over an access channel (the RACH or Random Access Channel in theGSM system). All of the mobile stations to which the system respondsfavorably for such allocation requests are associated with a sharedtraffic channel, and a virtual circuit is made available to each of them(unlike the real circuit made available to a mobile station to which atime slot has been allocated with no sharing for the entire duration ofa call).

There are several known techniques for determining which of the mobilestations associated with the same shared traffic channel can actuallytransmit data.

A first known technique of allocating the right to transmit dataoperates as follows: the base station indicates to all of the associatedmobile stations whether or not the shared traffic channel in the updirection is free. If it is free, a mobile station which wishes totransmit data to the base station sends an access request over theshared traffic channel in the up direction. The base station replies inthe down direction by authorizing the mobile station to transmit dataover the shared traffic channel in the up direction. The mobile stationthen monopolizes the traffic channel until it has no more data totransmit.

In the down direction, packet mode poses no problem: the base stationplaces, in the shared time slot contained in each frame that ittransmits, data addressed to a mobile station together with anidentifier for identifying the mobile station making it possible todistinguish it unambiguously from the other mobile stations associatedwith the same shared time slot.

Unfortunately, in the up direction, packet mode suffers from severaldrawbacks.

Firstly, when two mobile stations try to access the shared trafficchannel (or request authorization to transmit data) at the same timethere is a collision. The two mobile stations must then try again, e.g.after a random lapse of time. The number of mobile stations must remainlimited so as to reduce the number of collisions, and thus to make iteasier and quicker for mobile stations to access a shared time slot.

Furthermore, clearly a mobile station which has a large quantity of datato transmit monopolizes the shared time slot for a long time. This isunacceptable for the other mobile stations associated with the sharedtime slot, and waiting for it to become free so that they can transmitdata.

A second known technique, which attempts to mitigate the drawbacks ofthe first known technique, consists in granting access to a shared timeslot successively and after a fixed lapse of time to all of the mobilestations associated with the shared time slot, i.e. to each of themobile stations which, after it has made an allocation request, has beenallocated the shared time slot for the entire duration of its call.

That second known technique does prevent a mobile station frommonopolizing the shared time slot. Unfortunately, it suffers from thedrawback of granting access to the shared time slot to associated mobilestations even at times when those mobile stations have nothing totransmit. In other words, the use of the transmission resources is notoptimized.

A particular object of the invention is to mitigate the variousdrawbacks of the state of the art.

More precisely, an object of the present invention is to provide apacket access method in a cellular digital radiocommunications systemimplementing a TDMA technique, which method requires no modification tobe made to the frame structure of the system in which it is implemented.

Another object of the invention is to provide such a method whoseimplementation requires no additional transmission resources.

Another object of the invention is to provide such a method which, inthe presence of large amounts of data traffic, continues to offer highperformance in terms of resource sharing.

Another object of the invention is to provide such a method which makesit possible to limit the number of access collisions, and thus to reducethe time lapses between data transmission periods for the same mobilestation associated with a shared traffic channel.

Yet another object of the invention is to provide such a method makingit possible to monitor the traffic flow at the mobile station.

These various objects, and others that appear below are achievedaccording to the invention by means of a method of performing cellulardigital radiocommunications in a time division multiple access systemimplemented within a network of geographical cells through which mobilestations travel, a base station being associated with each of the cells,a mobile station communicating via the base station that is associatedwith the cell in which the mobile station is located;

the system conveying signals organized in frames, each of which isconstituted by N time slots, between the base station associated with acell and the mobile stations located in that cell, it being possible toassociate each time slot with a distinct call between the base stationand one of the mobile stations;

each of the frames in the up transmission direction, from the mobilestations to the base station, containing at least one shared time slotwhich is allocated to at least two distinct mobile stations;

the system having a list of the active mobile stations for each sharedtime slot, an active mobile station being a mobile station to which theshared time slot is allocated, and satisfying at least one predeterminedselection criterion;

and, for each frame in the up transmission direction, the systemassociating each shared time slot with a mobile station chosen from thelist of the active mobile stations associated with the shared time slot,the choice being made by using a predetermined strategy.

The principle of the invention is thus to distinguish those of themobile stations associated with each shared time slot that are activefrom those that are not, and to guarantee a minimum service (in terms oftransmission time) to the active mobile stations. In other words, ineach set of mobile stations to which the same shared time slot has beenallocated, a subset of active mobile stations is established.

Thus, as a function of a predetermined strategy, the system actuallygrants access to a shared time slot, e.g. successively, to each of theactive mobile stations on the list associated with the shared time slot,and not to all of the mobile stations associated with the shared timeslot (i.e. having obtained authorization to use it).

In this way, the use of transmission resources is optimized since onlythe active mobile stations actually share the same traffic channel (i.e.the same shared time slot). In other words, when a mobile station has nomore data to transmit but its call is not ended, it remains associatedwith the shared time slot but it is removed from the list of activemobile stations. As a result, the system no longer actually grants itaccess to the shared time slot, so as to avoid leaving the shared timeslot empty (i.e. not transmitting any data).

The method of the invention continues to offer high performance in thepresence of large amounts of traffic. The system actually grants accessto the shared traffic channel successively to each of the active mobilestations. As a result, an active mobile station that has a largequantity of data to transmit does not monopolize the shared trafficchannel for a very long length of time, but rather it is allocatedvarious access periods between which other active mobile stations cantransmit data.

The strategy for choosing an active mobile station from the list may bedefined in numerous ways. For example, a conventional polling techniquecan be used consisting in actually granting access to the shared trafficchannel successively and for a fixed length of time to each of theactive mobile stations on the list. It is also possible to makeprovision for the actual access time of each active mobile station to beof variable length, for certain active mobile stations to be actuallygranted access more frequently, etc.

The method of the invention also makes it possible to:

prevent collisions on the shared traffic channel, since the mobilestations do not send transmit authorization requests over this channel;and to

reduce collisions on the access channel since the mobile stations sendonly allocation requests over this channel (to enter the list for thefirst time), and, possibly, requests for authorization to transmit (tore-enter the list again if they have been removed from it).

Each mobile station thus makes only a few access attempts via the accesschannel. Therefore, the number of mobile stations can be high withoutcollision risks being too high.

It should be noted that the method of the invention can be implementedwithout making any modification to the frame structure, and withoutadding additional transmission resources (in particular control channelsand access channels).

Preferably, said list of active mobile stations is managed as a functionof at least one of the following predetermined selection criteria:

a mobile station is inserted into the list of the active mobile stationswhen it wishes to transmit data;

a mobile station is removed from the list of the active mobile stationswhen, after being allocated a determined number of access periods, ithas transmitted no data; and

a mobile station that has been removed from the list of the activemobile stations may be put back onto the list of the active mobilestations if it wishes to transmit data once again.

Thus, a mobile station associated with a shared time slot is either onthe list or off the list, and when it is off the list, it can be putback onto it if necessary.

Advantageously, the predetermined strategy used by the system to choosean active mobile station consists in allocating an access periodsuccessively to each of the active mobile stations on the list. In thisway, a minimum service is guaranteed to each active mobile station onthe list.

In a first advantageous implementation of the invention, the lapse oftime between two access periods for any one active mobile station isfixed, the maximum duration of each of the access periods being afunction of the number of active mobile stations on said list.

In a second advantageous implementation, the maximum duration of each ofthe access periods is fixed, the lapse of time between two accessperiods for any one active mobile station being a function of the numberof active mobile stations on said list.

Clearly, if it has nothing else to transmit, the mobile station can stoptransmitting data before the end of the maximum duration of the accessperiod allocated to it. In this case, the shared time slot may be usedby another mobile station.

Preferably, the list of the active mobile stations is managed in thebase station.

In a preferred implementation of the invention, a mobile station becomesactive when it receives a favorable reply from the system in response toa request for allocation of a shared time slot, which request is sent bythe mobile station to the system over a time slot that is dedicated toan access channel. Thus, in a GSM system, the access channel is a RACH.

Advantageously, a mobile station becomes active again by sending arequest for authorization to transmit data, the request being sent tothe system over a time slot that is dedicated to an access channel.

Advantageously, the system monitors the congestion of said accesschannel, and, as a function of this congestion of the access channel,the system modifies the predetermined number of access periods afterwhich a mobile station is removed from the list of the active mobilestations if it has transmitted no data.

In this way, it is possible to limit the overload on the access channelby keeping the active mobile stations longer on the list, even if theydo not transmit data. It is possible to reduce the number of transmitauthorization requests coming from mobile stations that wish to becomeactive again.

Preferably, when a mobile station becomes active for the first time, thesystem takes account of the number of active mobile stations alreadyassociated with the various shared time slots, when choosing the sharedtime slot with which the new active mobile station is to be associated.

In this way, the system takes account not only of the number of mobilestations to which each shared time slot has already been allocated, butalso of the number of these associated mobile stations that are active.For example, if the proportion of active mobile stations is low comparedwith the total number of mobile stations associated with a shared timeslot, the system may decide to continue to authorize access by newmobile stations to the shared time slot.

Advantageously, an active mobile station on a first list associated witha first shared time slot may be dynamically transferred to a second listassociated with a second shared time slot.

The number of active mobile stations makes it possible to know the upperand lower limits for the amount of traffic that a base station canhandle. It is therefore possible to monitor the traffic flow at the basestation, and, where necessary, to decide to transfer it dynamically fromone list to another, i.e. to re-allocate some other shared time slot toan active mobile station.

Advantageously, an active mobile station is dynamically transferred froma first list to a second list if at least one of the followingconditions is satisfied:

the number of active mobile stations on the first list is greater than apredetermined threshold number; and

the active mobile station has a quantity of data to be transmitted thatis greater than a predetermined threshold quantity.

If an active mobile station transmits data in the form of data blocks,then advantageously an active mobile station that is in the process oftransmitting adds a first item of information to each of said datablocks so as to indicate whether or not it still has data to betransmitted.

Preferably, at least two data blocks from the same active mobile stationare interleaved simultaneously over at least two shared time slotsassociated with the same active mobile station, at a rate of one sharedtime slot per frame in the up direction;

and each shared time slot is divided into at least two portions, each ofwhich receives a portion of one of the simultaneously interleavedblocks.

In this case, advantageously, the first item of information is added toa single one of the simultaneously interleaved data blocks. In this way,if the data blocks are of fixed length, additional items of informationcan be added instead of the first item of information in thosesimultaneously interleaved data blocks to which the first item ofinformation has not been added.

Similarly, if the system transmits data in the form of data blocks inthe down direction, then advantageously as a function the predeterminedstrategy for making a choice, the system adds the following to each ofthe data blocks:

a second item of information indicating an active mobile station on thelist to which the next access period in the up transmission direction isallocated; and

a third item of information indicating an active mobile station to whichthe data block is addressed.

These second and third items of information are added at a base stationsub-system to which the base station belongs.

Preferably, at least two data blocks in the down direction that areaddressed to the same mobile station are interleaved simultaneously overat least two time slots associated with the same mobile station, at arate of one time slot per frame in the down direction;

and each shared time slot is divided into at least two portions, each ofwhich receives a portion of one of the simultaneously interleavedblocks.

In this case, advantageously, at least one of the second and third itemsof information is added to a single one of said simultaneouslyinterleaved data blocks. In this way, if the data blocks are of fixedlength, additional items of information can be added instead of at leastone of the second and third items of information in those simultaneouslyinterleaved data blocks to which at least one of the second and thirditems of information has not been added.

Other characteristics and advantages of the invention appear on readingthe following description of a preferred embodiment of the invention,given by way of non-limiting example and with reference to theaccompanying drawings, in which:

FIG. 1 diagrammatically shows an example of a cellularradiocommunications system in which the invention can be implemented;

FIG. 2 shows an example of the structure of a frame and the use of itwith a method of the invention;

FIG. 3 diagrammatically shows an example of a list of active mobilestations from a set of mobile stations associated with the same trafficchannel in accordance with the method of the invention;

FIG. 4 shows an example of a strategy for choosing an active mobilestation for a shared time slot;

FIG. 5 shows an example of a flow chart of the method of the invention;

FIGS. 6 and 7 show examples of data blocks, transmitted respectively inthe up direction and in the down direction;

FIG. 8 diagrammatically shows an example of how data blocks may beinterleaved, corresponding to a particular implementation of the methodof the invention; and

FIGS. 9 and 10 show examples of transmitting additional data in datablocks intended to be interleaved as shown in FIG. 8, for data blocksrespectively in the up direction and in the down direction.

The invention relates to a packet access method in a cellular digitalradiocommunications system.

In the remainder of the description, the system is assumed to be of thetype designed in compliance with the GSM public radio telecommunicationsstandard. Clearly, however, the invention is not limited to thisparticular type of system.

FIG. 1 diagrammatically shows an example of such a radiocommunicationssystem. The territory covered by the radiocommunications system isdivided up into cells 111 to 11n. Each cell 111, 115 includes a basestation 121, 125 capable of interchanging signal frames 131, 132 with aplurality of mobile stations 141, 142 travelling in the cell 111.Frequency allocation is based on organizing the cells in patterns 151,152 of seven cells. The cells in any one pattern use distinct frequencybands. In contrast, the frequency bands are re-used from one pattern 151to another 152.

FIG. 2 shows an example of a frame structure when theradiocommunications system implements a time-division multiplexingtechnique of the TDMA (Time Division Multiple Access) type.

The frame 21 is made up of N time slots IT1 to IT8 (N=8 in thisexample). Each time slot, of duration T, can be associated with arespective transmission channel. In the example shown in FIG. 2, a timeslot IT1 is associated with an access channel (RACH, for Random AccessChannel) and each of the other time slots IT2 to IT8 is associated witha respective traffic channel (TCH, for Traffic Channel).

The time slots IT2 to IT8 associated with respective traffic channelsare allocated using either circuit mode or packet mode. In any oneframe, both allocation modes (circuit mode and packet mode) can existsimultaneously. Some time slots are then allocated in circuit mode andothers are allocated in packet mode.

In circuit mode, a time slot IT2 to IT5, IT7, IT8 is allocated to amobile station SM1 to SM4, SM5, SM6 for the entire duration of the callof the mobile station. Each mobile station thus has its own trafficchannel TCH1 to TCH4, TCH5, TCH6.

In packet mode, a time slot IT6 is allocated to a plurality of mobilestations SM7 to SM22 which thus share the corresponding traffic channelTCHP. Such a time slot or traffic channel may be referred to as a sharedtime slot or a shared traffic channel.

The invention more precisely concerns an access protocol method foraccessing a shared time slot.

According to the method of the invention, for each shared trafficchannel, the system has a list of active mobile stations, i.e. a list ofmobile stations which not only have been allocated the shared time slot,but also satisfy one or more selection criteria (e.g. not remaining toolong without having data to transmit).

A mobile station can be inserted into the list, removed from it, or putback onto it, as explained below with reference to FIG. 5.

In the up transmission direction (from a mobile station to the basestation), only the mobile stations on the list in fact have access tothe shared traffic channel TCHP. In other words, only the active mobilestations share the shared traffic channel TCHP. Thus, as showndiagrammatically in FIG. 3, the list 31 of the active mobile stationsSM7, SM8, SM10, SM12, SM13, SM16, SM17, SM19, SM22 associated with ashared traffic channel TCHP is a subset of the set 32 of the mobilestations SM7 to SM22 to which the shared traffic channel TCHP has beenallocated.

For each successive frame in the up direction, the system chooses anactive mobile station from the list using a predetermined strategy. FIG.4 shows an example of such a selection strategy, with the list of activemobile stations as shown in FIG. 3. In this example, the system actuallygrants access to the shared time channel successively to each of themobile stations on the list. The access period of a mobile station tothe shared traffic channel (i.e. a period during which it can transmitdata) ends either when the mobile station has no more data to transmit,or when the maximum duration is reached. In the latter case, the mobilestation must wait for its next access period before it continues totransmit its data.

In the example shown in FIG. 4, the lapse of time T1 between two periodsof access for any one active mobile station is fixed. Therefore, themaximum duration TM of each access period is a function of the number ofactive mobile stations on the list.

In a variant (not shown), it is the lapse of time between two accessperiods for any one active mobile station that is fixed. The maximumduration of each access period is then a function of the number ofactive mobile stations on the list.

FIG. 5 shows an example of a flow chart of the method of the invention.

When a mobile station wishes to transmit data for the first time, itsends (51) a request for allocation of a shared time slot over theaccess channel.

If another mobile station transmits a request at the same time, there isa collision (52), and the two mobile stations must try again.

If there is no collision, the system records the request and allocates ashared time slot to the mobile station: this mobile station is put (53)on the list of active mobile stations associated with the shared timeslot. In addition, an identifier is associated with each mobile stationat the time of allocation, so as to distinguish it unambiguously fromthe other mobile stations associated with the same shared time slot. Inother words, each of the mobile stations associated with any one sharedtime slot has a distinct identifier. In contrast, two mobile stationsassociated with two distinct shared time slots can have the sameidentifier.

Then, the mobile station (54) waits for the system to allocate it aperiod of access to the traffic channel corresponding to the shared timeslot that has been allocated to it.

Once the mobile station has been allocated an access period (55),various possible situations can occur depending on whether or not themobile station has data to transmit (56).

If the mobile station has data to transmit, it transmits it (57) duringthe access period, and it then waits (54) for a new access period(except, of course, if its call is over).

If the mobile station has no data to transmit, the system increments(58) by one the number N of successive access periods allocated to themobile station without it transmitting data, the system then compares(59) the incremented number with an optionally variable determinedthreshold number N_(s). If N<N_(s), the mobile station waits (54) for anew access period (and it remains on the list). If N≧N_(s), the mobilestation is removed (510) from the list of the active mobile stations.

It is possible to consider indicating to the mobile station that it hasreached its last try before it is removed from the list, so that themobile station knows exactly as from when it is no longer on the list.Otherwise, a mobile station can know that it is no longer on the listonly if, after a certain lapse of time, the system does not actuallygrant it access again to the shared traffic channel. By enabling themobile station to know exactly when it is removed from the list, thesystem avoids making the mobile station wait unnecessarily for an accessperiod. In this way, when the mobile station is removed from the listand wishes to transmit data again, it immediately makes a request forauthorization to transmit so as to be put back onto the list (withoutwaiting, believing that it is still on the list, for the system to grantit actual access to the shared traffic channel).

When it is informed that it has reached its last try before it isremoved from the list, a mobile station may optionally transmitarbitrary data for the sole purpose of remaining on the list.

After it has been removed from the list, if the mobile station has datato transmit (511), it can be put back onto the list if it transmits (51)a request for authorization to transmit data (indicating which time slothas already been allocated to it).

The method of the invention may have numerous other characteristics. Forexample, provision may be made for the system to modify the thresholdnumber Ns after which a mobile station is removed from the list ofactive mobile stations if it has transmitted no data, as a function ofthe congestion of the access channel (RACH).

To decide which time slot to associate with a mobile station, the systemmay also take account of the number of mobile stations that are active(and not merely associated) for each of the shared time slots.

It is further possible to transfer dynamically a mobile station betweenshared time slots, e.g. if a mobile station has a large quantity of datato transmit and/or if there are a large number of active mobile stationsin the list to which the mobile station belongs.

It is also possible to provide priority levels within the list of activemobile stations. For example, a mobile station that has just entered thelist may be considered as being higher priority (so that it is actuallygranted access quicker) than an active mobile station which hastransmitted nothing after several access periods.

The data transmitted by a base station or by an active mobile station isgenerally in the form of data blocks. The remainder of the descriptionis more precisely concerned with examples of data blocks and aparticular implementation of interleaving the data blocks.

FIG. 6 shows an example of a data block transmitted in the up direction.The data block 61 is constituted by a "conventional" data block 62 towhich a first item of information 63 is added. The term "conventional"data block 62 refers, for example in GSM, to an RLP block (or frame)that can be contained in four time slots, and that contains a header,message data, and a frame control sequence. The first item ofinformation 63 makes it possible for the mobile station to indicate tothe base station whether or not it still has data to transmit. One bittherefore suffices for the first item of information.

FIG. 7 shows an example of a data block transmitted in the downdirection. This data block 71 is constituted by a "conventional" datablock 72 (as above) to which a second item 73 and a third item 74 ofinformation have been added (e.g. in the base station sub-system of thebase station).

The second item of information 73 enables the system to indicate whichactive mobile station will be allocated the next period of access to theshared time slot.

The third item of information 74 is a mobile station identifier for agiven shared time slot. This identifier (described above) enables thesystem to indicate to which active mobile station (from the activemobile stations associated with the given shared time slot) the datablock 71 is addressed.

Thus, with a length of four bits for each of the second and third itemsof information, the system can grant one mobile station out of sixteenactual access to the shared time slot (by means of the second item ofinformation), and send data to one mobile station out of sixteen (bymeans of the third item of information).

FIG. 8 diagrammatically shows an example of interleaving data blocks (inthe up direction or in the down the direction).

In this example, each frame FRAME 1 to FRAME 16 comprises eight timeslots IT1 to IT8 of duration T, including a shared time slot IT6. Fourdata blocks BLOCK 1 to BLOCK 4 in the same call are interleavedsimultaneously ("rectangular" interleaving) over sixteen consecutiveframes FRAME 1 to FRAME 16. For this purpose, the shared time slot IT6of each frame is divided up into four and it receives one sixteenth ofeach of the four data blocks BLOCK 1 to BLOCK 4.

Clearly, the rectangular interleaving may be implemented differently.For example, two data blocks may be interleaved over eight frames, bydividing up each shared time slot into two. Similarly, other lengths ofdata block may be envisaged (as a function of the encoding ratio).

As shown in FIG. 9, for the data blocks in the up direction, and in FIG.10, for the data blocks in the down direction, rectangular interleavingmakes it possible, in the case of the invention, to transmit additionalinformation.

The simultaneously interleaved blocks belong to the same call.Therefore, the item(s) of information 63, 73, 74 to be added to the"conventional" data blocks 62, 72, may be added to a single data blockin each group of simultaneously interleaved blocks.

Thus, in the up direction, as shown in FIG. 9, a data block BLOCK 1contains the first item of information 63, and the other blocks BLOCK 2to BLOCK 4 have empty spaces 91 for transmitting additional information.

Likewise, in the down direction, as shown in FIG. 10, a data block BLOCK1 contains the second and third items of information 73, 74, and theother blocks BLOCK 2 to BLOCK 4 have empty spaces 101 for transmittingadditional information.

In the up direction, the additional information 91 is, for example,signalling data.

In the down direction, the additional information may be identifierover-encoding data, or data enabling the traffic flow to be monitored atthe mobile station: for example, the base station indicates to themobile station how many mobile stations are active, which enables themobile station to know how much transmission time is available to it; orelse, the base station warns the mobile station that it is actuallygoing to be removed from the list, thereby enabling the mobile stationto send data pre-emptively (so as to remain on the list and avoid havingto send a request over the RACH for authorization to transmit data overthe shared traffic channel).

The invention is described with reference to a cellularradiocommunications system because optimizing the use of the trafficchannels is particularly relevant to such systems. However, theinvention is clearly applicable to any type of transmission system,regardless of the transmission medium. In particular reference may bemade to non-cellular radio networks (particularly in ISM (Industrial,Scientific, and Medical) frequency bands), cable networks, andoptical-fiber networks, whether they cover large areas or whether theyare local networks.

The invention offers a general solution to the problem posed by managinga channel shared between a plurality of users.

I claim:
 1. A method of performing cellular digital radio-communicationsin a time division multiple access system implemented within a networkof geographical cells through which mobile stations travel, a basestation being associated with each of the cells, at least one of themobile stations communicating via the base station that is associatedwith the cell in which the one mobile station is located;the systembeing configured to convey signals organized in frames, each of which isconstituted by N time slots, between the base station associated with acell and the mobile stations located in that cell, and configured toassociate each time slot with a distinct call between the base stationand one of the mobile stations; each of the frames in an up transmissiondirection, from the mobile stations to the base station, containing atleast one shared time slot which is allocated to at least two distinctones of the mobile stations; said method comprising steps of:for eachshared time slot, providing a list of active mobile stations, an activemobile station being any one of the mobile stations to which the sharedtime slot is allocated and which additionally satisfies at least onepredetermined selection criterion; and for each frame in the uptransmission direction, associating the shared time slot with a specificone of the mobile stations selected from the list of the active mobilestations, the selection being based on a predetermined strategy.
 2. Themethod according to claim 1, wherein said at least one predeterminedselection criterion for providing the list of the active mobile stationscomprises at least one of the following criteria:a given one of themobile stations is inserted into the list of the active mobile stationswhen the given station wishes to transmit data; a certain one of theactive mobile stations is removed from the list of the active mobilestations when, after being allocated a determined number of accessperiods, the certain station has transmitted no data; and a particularone of the mobile stations that has been removed from the list of theactive mobile stations is put back onto the list of the active mobilestations if the particular station wishes to transmit data once again.3. The method according to claim 1, wherein said predetermined strategyfor selecting the specific active mobile station comprises allocating anaccess period successively to each of the active mobile stations on thelist.
 4. The method according to claim 3, wherein a lapse of timebetween two access periods for any one of the specific selected activemobile stations is fixed, a maximum duration of each of the accessperiods being a function of a total number of the active mobile stationson the list.
 5. The method according to claim 3, wherein a maximumduration of each access period for any one of the specific selectedactive mobile stations is fixed, a lapse of time between two of theaccess periods for the one selected specific active mobile station beinga function of a total number of the active mobile stations on the list.6. The method according to claim 1, wherein the list of the activemobile stations is managed in the base station.
 7. The method accordingto claim 1, wherein a given one of the mobile stations becomes activewhen the given station receives a favorable reply from the system inresponse to a request for allocation of a shared time slot, whichrequest is sent by the given mobile station to the system over a timeslot that is dedicated to an access channel.
 8. The method according toclaim 7, wherein the given mobile station, following deactivation,becomes active again by sending a further request for authorization totransmit data, the further request being sent to the system over thetime slot that is dedicated to an access channel.
 9. The methodaccording to claim 7, wherein the system monitors congestion of theaccess channel, and wherein, as a function of the congestion of theaccess channel, the system modifies a value indicative of a number ofaccess periods after which the given mobile station is removed from thelist of the active mobile stations if the given station has transmittedno data.
 10. The method according to claim 1, wherein, when a given oneof the mobile stations becomes active a first time, the system takesaccount of a total number of active mobile stations already associatedwith a total number of available shared time slots, when allocating theshared time slot with which the given active mobile station is to beassociated.
 11. The method according to claim 1, further comprisingdynamically transferring a particular one of the active mobile stationson the list associated with a first one of the shared time slots to thelist associated with a second one of the shared time slots.
 12. Themethod according to claim 11, wherein the particular active mobilestation is dynamically transferred from the first list to the secondlist if at least one of the following conditions is satisfied:a totalnumber of the active mobile stations on the first list is greater than apredetermined threshold number; and the particular active mobile stationhas a quantity of data to be transmitted that is greater than apredetermined threshold quantity.
 13. The method according to claim 1,wherein the active mobile stations transmit data in the form of datablocks and wherein a particular one of the active mobile stations thatis in the process of transmitting adds a first item of information toeach of the data blocks the particular station is transmitting so as toindicate whether or not the particular station still has data to betransmitted.
 14. The method according to claim 13, wherein at least twoof the data blocks from the particular active mobile station areinterleaved simultaneously over at least two of the shared time slotsassociated with the particular active mobile station, at a rate of oneshared time slot per frame in the up transmission direction;and whereineach shared time slot is divided into at least two portions, each of theportions receiving a portion of each of the at least two simultaneouslyinterleaved data blocks.
 15. The method according to claim 14, whereinthe first item of information is added to a single one of thesimultaneously interleaved data blocks.
 16. The method according toclaim 15, wherein the data blocks are of fixed length, and whereinadditional items of information other than the first item of informationare added to the simultaneously interleaved data blocks other than thesingle data block to which the first item of information has been added.17. The method according to claim 1, the system transmitting data in theform of data blocks in a down transmission direction, wherein, as afunction of said predetermined strategy for the system adds thefollowing to each of the data blocks:a second item of informationindicating a given one of the active mobile stations on the listselected for a next access period in the up transmission direction; anda third item of information indicating a particular one of the activemobile stations to which the data block is addressed.
 18. The methodaccording to claim 17, wherein the second and third items of informationare added at a sub-system to which the base station belongs.
 19. Themethod according to claim 17, wherein at least two of the data blocks inthe down transmission direction that are addressed to the particularmobile station are interleaved simultaneously over at least, two of theshared time slots associated with the particular mobile station, at arate of one time slot per frame in the down transmission direction;andwherein each shared time slot is divided into at least two portions,each of of the portions receiving a portion of each of the at least twosimultaneously interleaved data blocks.
 20. The method according toclaim 19, wherein at least one of the second and third items ofinformation is added to a single one of the simultaneously interleaveddata blocks.
 21. The method according to claim 20, wherein the datablocks are of fixed length, and wherein additional items of informationother than at least one of the second and third items of information areadded to the simultaneously interleaved data blocks other than thesingle data block to which the at least one item of information has beenadded.
 22. A method for transmitting data in a signal frame comprising aplurality of time slots, at least one of the time slots being a sharedtime slot allocated to a total number of at least two distinct mobilestations, comprising:utilizing a predetermined selection criterion toproduce a list of active mobile stations from the total number of themobile stations; selecting only one mobile station from the list ofactive mobile stations in accordance with a predetermined strategy; andtransmitting the data in the shared time slot via the selected mobilestation.
 23. The method according to claim 22, wherein the list containsless then the total number of mobile stations.